Method of electrical discharge machine with simultaneous relative advance and cyclic translational movement of the electrodes

ABSTRACT

In the electrical discharge machining of the surface of a recess in a workpiece electrode to a shape corresponding to that of a tool electrode, a method whereby the electrodes are relatively displaced, during a finishing phase of machining, along an axis of penetration and simultaneously in a plane perpendicular to the axis of penetration with a cyclic translational movement whose amplitude increases with the penetration of the tool in the workpiece. This provides a virtual 3-dimensional dilatation of the tool as it advances, and enables control of the sparking in both the frontal and lateral parts of the machining zone such that the shape of the machined portion is an image of the tool shape. The cyclic translational movements can be obtained with an eccentric member whose eccentricity relative to a shaft is controlled by limiting axial movement of the shaft at a position corresponding to a predetermined penetration.

CROSS-REFERENCE TO RELATED APPLICATION

This application is related to application Ser. No. 696,712, filed onthe same date as the present application and assigned to the sameassignee.

BACKGROUND OF THE INVENTION

The invention concerns the electrical discharge machining of the surfaceof a recess in a workpiece electrode by means of a tool-formingelectrode. For the sake of simplification, these electrodes willhereinafter be referred to separately as "workpiece" and "tool" andjointly as "electrodes." The invention specifically concerns machiningin which the electrodes are moved relative to one another both in thedirection of an axis of penetration of the tool in the workpiece and ina plane perpendicular to this axis.

It is known to machine the lateral faces of a workpiece by moving theelectrodes relative to one another with a translational movement, ororbital motion, in a plane perpendicular to the axis of penetration asdisclosed in U.S. Pat. No. 2,773,968, and vary the amplitude of thismovement as a function of the relative displacements of the electrodesalong said axis as disclosed in U.S. Pat. Nos. 3,135,852, 3,539,754 and3,809,852, which displacements are controlled to maintain given sparkingconditions in the machining zone comprised between the electrodes.

All those methods of machining, result in machining of the lateralsurfaces of the workpiece to a shape different from the shape of thetool. Another result is that the active lateral surface of the tool isdisplaced substantially parallel to the machined surface on theworkpiece, such that there is no increase of the gap when the toolelectrode is retracted. The machined shape, which is for exampledetermined by the factor of proportionality between the amplitude of thetranslational movement and the axial displacement of the tool,corresponds to the shape of an envelope of the trajectories during thecyclic translational movements. A similar method, in which an orifice ofincreasing conicity is machined with a tool in the form of a flat disc,is described in W.German Published Patent Application (DOS) No. 2 238698.

The invention concerns another method of machining which enablessimultaneous machining of the front and lateral surfaces of a recess tothe same shape as the tool. Up to the present, the finishing machiningof a recess by successive passes with a single tool could be carried outusing the method described in French Pat. No. 1 274 953. This methodconsists in making the tool penetrate in the workpiece during amachining pass by a relative transverse translational displacement ofconstant amplitude, and then repeating this operation, in a followingpass, after having increased the amplitude of this movement, thususually causing at least a partial withdrawal of the tool from theworkpiece. This method of machining produces a great local wear of thetool and necessitates a control of the frontal penetration of the toolin the workpiece in addition to control of the amplitude of thetranslational movement.

The invention provides a new method which enables; (a) avoidance ofsuccessive withdrawals of the tool from the workpiece, (b) eliminationof local wear of the tool, and (c) control of the three-dimensionalprogression of machining, using a single device for measuring theamplitude of the translational movement.

This new method of machining is characterized in that one of theelectrodes is made to penetrate into the other to a depth such thatsparking is produced on the frontal and/or lateral parts of saidsurface, then the electrodes are moved relative to one another withoblique translational movements along at least one generatrix of asurface of revolution of increasing section in said direction ofpenetration, these oblique translational movements being obtained in amanner known per se by a rigid connection between rectilineartranslational movements along said axis and translational movement insaid plane and controlled, in a manner known per se, so as to maintaingiven sparking conditions in a machining zone comprised between theelectrodes, this zone simultaneously extending on said frontal andlateral parts of said surface during at least a part of the finishingmachining.

When, for example, said surface of revolution is in the form of a conewhose apex forms a right angle, control of the axial and radial advanceof machining is obtained simply by a limitation of the amplitude of thetranslational movement to a predetermined value. This limiting valuecorresponds to a given point of one of the generatrices of this cone inthe case of linear translation, or to a circle inscribed on the surfaceof this cone when there is a cyclic translation. In both cases, therapidity and precision of machining are greater than those that would beobtained using the known machining methods.

SUMMARY OF THE INVENTION

The method of the present invention has for one of its objects to formin a workpiece an EDM machined portion which has substantially the samegeometry or shape as that of the tool electrode, by causing for allpractical purposes a 3-dimensional dilatation of the tool electrode,which is obtained by varying the eccentricity of the translation motionof the tool electrode continuously of an amount proportional to therelative axial displacement of the electrode.

One of the advantages obtained by the new method of the invention is toeffectuate and simultaneously control the frontal and lateral machiningin the course of roughing and finishing passes, and to provide an equalwear of the tool electrode on all its active surfaces.

The many objects and advantages of the present invention will becomemore apparent to those skilled in the art when the following descriptionof the best modes contemplated for practicing the invention is read inconjunction with the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING

The accompanying drawings show, schematically and by way of example, twoembodiments of a device for carrying out the method according to theinvention.

In the drawings:

FIG. 1 is a partial view in longitudinal cross-section of a firstembodiment;

FIG. 2 is a view along arrow A of FIG. 1, partly cut away and incross-section;

FIG. 3 is a partly cut-away side elevational view of a secondembodiment.

FIG. 4 is an end elevational view of the device of FIG. 3;

FIG. 5 is a cross-section along line V--V of FIG. 4; and

FIG. 6 is a cross-section along line VI--VI of FIG. 5.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The device A of FIGS. 1 and 2 is carried mainly by a plate 2 fixed to apiston 1 of an electrical discharge machine. This device is connected toa bracket 5 of the machine by two threaded rods 3 engaging in settingnuts 4. The device A is generally symmetrical about a plane of symmetry6, only its left hand part being shown in FIG. 1. A second rod,identical to rod 3, is thus disposed to the right of the machine and isnot shown in the drawings.

During finish machining, the piston 1 is moved axially. The plate 2carries a block 7 enclosing a gear mechanism and provided with anelectrode support 8. Piston 1, plate 2 and block 7 form a compact unitin relation to which the electrode support 8 can move in translation inits horizontal plane. This displacement of support 8 in relation to theblock 7 is made possible by tie-bolts 9 which each pass through a boreof block 7 with a play 10. The support 8 is connected to block 7 by thetie-bolts 9 with interposed abutment bearings 11 allowing displacementof the support 8 in relation to block 7 within the limits of play 10.The support 8 has screw-threaded openings 12 for receiving boltssecuring an electrode, not shown.

The parts of the device A which serve to produce translationaldisplacements of the support 8 in relation to block 7 will now bedescribed.

A hollow guide shaft 13 passes through the block 7 and electrode support8. This shaft 13 has a longitudinal groove 14 having a terminal partforming a ramp 15. The shaft 13 turns freely about a rod 16 which passesthrough the shaft 13 and is threaded at least at each end. Rod 16 isfixed in a piece 17 connected to the threaded rod 3. Piece 17 has aportion 18 in which rod 3 is secured by a transverse pin 19. Thus, rod3, portion 18, piece 17 and rod 16 form a rigid assembly in thelongitudinal direction.

At the lower end of rod 16 is a nut 20 axially fixing the shaft 13 tosaid rigid assembly, while allowing it to turn freely on two axialball-bearings 21 and 22.

The piece 17 has an external thread screwed in a setting nut 23 bearingon a cap 24 secured on block 7 by screws. Hence, all of the elements ofthe device, in as much as they are not supported by bracket 5, arecarried by the plate 2.

The block 7 is provided with two combined axial and radial needlebearings 25 and 26 on which a sleeve 27 is pivotally mounted. Bearings25 and 26 are set by a nut 28. The shaft 13 is slidably mounted insleeve 27 by means of a ball-slide 29, known per se. It is hencepossible to control an axial movement of the shaft 13 in sleeve 27either by nut 23 or by nut 4, as will be explained further on.

The electrode support 8 has a housing 30 in which is disposed a radialneedle-bearing 31 on which an eccentric 32 is pivotally mounted. Theeccentric 32 is annular and surrounds the shaft 13 with a play 33 whichmay be equal to play 10. The upper part of eccentric 32 and the lowerpart of sleeve 27 engage with one another by a radial slide 34 so thatthe eccentric 32 may move radially in relation to the sleeve 27 by theamount of play 33. This radial movement of the eccentric 32 iscontrolled by a roller 35 mounted on the eccentric for cooperation withramp 15. A spring 36, acting between the sleeve 27 and eccentric 32,biases the eccentric 32 to tend to keep it centred in relation to shaft13. The block 7 also houses transmission members including a bevelpinion 37 fixed on a toothed wheel 38 meshing with another toothed wheel39 which in turn meshes with an external toothing 40 on sleeve 27. Thebevel pinion 37 is rotatably driven by a bevel pinion 41 of an electricmotor 42. As the roller 35 of eccentric 32 is engaged in the slot 14 ofshaft 13, the latter is also rotatably driven by the eccentric 32. Whenthe eccentric 32 is disposed centrally about the shaft 13, i.e. withzero eccentricity, no displacement of the electrode support 8 relativeto block 7 is produced in response to rotation of the shaft 13 andeccentric 32.

The bracket 5 is provided with a sleeve 43 of insulating materialthrough which the threaded rod 3 passes with play. This sleeve 43carries the setting screw 4 by the intermediary of an axial ball bearing44. The transverse pin 19 is urged by two traction springs 46 so thatshaft 13 correctly bears against the bracket 5 fixed on the frame 45 ofthe machine. The springs 46 are indirectly connected to block 7. Asshown in FIG. 2, block 7 also carries a bracket 47 adapted to carry acomparator, not shown.

It has already been stated above that the device is generallysymmetrical about the plane 6. However, this symmetry does not apply tothe elements, 1, 2, 7, 8, 41, 42, 37, 38 and 45. In accordance with thissymmetry, the device comprises two eccentrics 32 with two slides 34parallel to one another for a given position of the eccentrics disposedon the same line. When the eccentric 32 is decentred in relation toshaft 13, the entire electrode support 8 moves as an eccentricconnecting rod.

In an example of carrying out the method according to the invention, thedevice of FIGS. 1 and 2 is used as follows:

Firstly, one proceeds to adjust the position of eccentrics 32 so thatthey have a zero eccentricity on their respective shaft 13 when the nut23 bears against the upper face of cap 24. This adjustment is carriedout by turning each nut 23 relative to piece 17 to axially displace theshaft 13 until the roller 35 lies on ramp 15 in a position giving zeroeccentricity. To carry out this adjustment, a comparator is placed oneach bracket 47 with a sensor of the comparator applied against theelectrode support 8. The motor 42 is started, and the describedadjustment is carried out until the comparator indicates that thesupport 8 is not deviated while the motor continues to rotate theeccentrics.

Then the desired depth of machining is adjusted by acting on nut 4. If,during this adjustment, the electrode rests on the upper face of theworkpiece, the depth of machining corresponds to the distance the nutmust travel to abut against the axial bearing 44, plus the displacementduring finishing machining and the spark distance during the finishingphase.

Rough machining is then carried out, by advancing the electrode in theconventional manner to make it penetrate in the workpiece. During thisfirst phase of machining, the nut 23 rests on the cap 24 so that theelectrode is not subjected to a translational movement since the devicehas a zero eccentricity. During this first phase, the piston 1 controlsthe advance of the electrode. As this advance progresses, the nut 4moves towards its position of abutment against the bracket 5 and, whenthis position is reached, the machining passes from the rough phase tothe finishing phase.

As soon as the nut 4 abuts against the bearing 44, any furtherdisplacement of the piston 1 towards the workpiece produces adisplacement of shaft 13 relative to the main part of the device andhence produces an increasing eccentricity of shaft 13. This eccentricityis very small and in general remains well below 1 millimeter.Consequently, the cavity machined in the workpiece has the same shape asthe electrode.

The rigid linkage which determines the degree of eccentricity of thetranslational movement as a function of the advance of the electrodefrom the moment when the nut 4 has reached its axial abutment positionproduces a virtual dilatation of the electrode in a plane perpendicularto the axial direction along which it advances. This arrangement is veryadvantageous since as soon as the machining conditions tend todeteriorate and it is consequently necessary to slightly withdraw theelectrode, this withdrawal is accompanied by a reduction of theamplitude of translation and hence of the virtual dilatation of theelectrode. Hence, the servo-control device, which operates solely in theaxial direction, simultaneously controls a radial withdrawal of theenvelope generated by the translational movement of the electrode. Thus,the servo-control device can operate to adjust the position of theelectrode as a function of the instantaneous machining conditions whiletaking into account the conditions of both the frontal machiningdistance and the lateral machining distance.

The embodiment of FIGS. 3 to 6 involves an improvement in that thedevice is of simpler construction without a reduction of the quality ofmachining.

In this embodiment, the device is not symmetrical about plane 6. Themachine has a frame 45 and a piston 1 carrying plate 2. There is only asingle control device, visible in the left hand part of FIG. 3. Theblock 7' is fixed to the plate 2 and a casing 55 is rigidly fixed, withan interposed insulating plate 56, to the plate 2. Electrode support 8'is connected to block 7' by a table with a cross-sliding arrangementincluding slides 48 and 49. The support 8' is rigidly connected to alower casing 58 with an interposed insulating plate 57. The casings 55and 58 are connected together by tie-bolts 9 having bearings 11 allowingrelative displacements of the casings in a horizontal plane, within thelimits of play 10.

A device for producing a translational movement of support 8' with avariable radius comprises a vertical rod 59 having a square portion 60as well as a threaded part 61 towards its lower end, as shown in FIG. 5.The lower end of rod 59 is mounted in two axial bearings 62 and 63 so asto be connected without axial play to a piece 64 while being able tofreely turn in relation to piece 64.

The upper end of piece 64 has a toothing 65 by which it can be rotatablydriven by a motor and gears, not shown, analogous to those of the firstembodiment. This piece 64 has a lower fork-shaped part having lateralbranches 66 and 67 (FIG. 6). These branches 66, 67 have, on their innerfaces, a part of two rectilinear guide devices 68.

The two guide devices 68 are inclined to the axis of rotation of piece64 at an angle of 45° in the example shown.

The piece 64 is placed in a sleeve 69 carrying a cage of balls 70forming a bearing with the casing 55 and allowing both rotation andaxial displacement of the piece 64 in casing 55. The rod 59 protrudesfrom casing 55 by its threaded portion 61 which carries an abutment nut71 which can be locked in position by a screw 72. For the purposes ofassembly, the casing 55 is formed of two parts, a hollow rectangularpart 73 and a cover 74.

An eccentric 75 of variable eccentricity is formed by a triangular plate76 sliding between the branches 66 and 67 along guide devices 68. Thisplate 76 carries two downwardly-protruding coaxial studs 77 and 78pivoting in casing 58 by the intermediary of a radial bearing 79 and twoaxial bearings 80.

To eliminate any axial play between the eccentric 75 and casing 58, anut 83 is screwed on a threaded terminal portion of stud 78. This nut 83is locked by a safety washer 82 and enables adjustment of the axialbearings 80 by the intermediary of a washer 81.

The eccentric 75 also has an axial bore in which is placed a settingscrew 84 forming a hooking point for an end of a spring 87 whose otherend is hooked on a pin 85 of piece 64. This spring 87 biases the rod 59and piece 64 downwards towards the eccentric 75.

The eccentric 75 is thus situated at the lower end of piece 64, and theguide devices 68 form a ramp between the piece 64 and eccentric 75. Anaxial displacement of piece 64 in relation to casing 55 produces aradial displacement of the eccentric 75 together with casing 58.

The upper part of rod 59 is smooth and engages in an opening of a casing88 fixed to the frame 45 of the machine. This rod 59 carries anadjustable piece 89 whose position on rod 59 can be set by a screw 90.Piece 89 has the same role as the nut 4 of FIG. 1, i.e. it determinesthe depth of penetration of the electrode from which machining should becarried out with translation of the electrode.

The casing 88 also contains a wedge 91 able to be moved towards theright, FIG. 3, by means of a setting screw 92 against the action of abiasing spring, not shown. A plate 93 is placed on wedge 91 and isapplied against it by two traction springs 94. These springs 94 arehooked at their lower ends on pins 95 held under the casing 88 and attheir upper ends on pins 96 bearing on the plate 93 which forms anabutment for piece 89.

The position of plate 93, set by the screw 92 and wedge 91, can bedetected by a comparator 98 having a feeler in contact with the end of ascrew 97 engaged in a threaded hole of plate 93.

Operation of the device of FIGS. 3 to 6 is very similar to that of thefirst embodiment.

The position of nut 71 can be set to provide a zero eccentricity of theeccentric 75 when the nut 71 bears on cover 74. Adjustment of theposition of piece 89 on rod 55 sets the distance by which the electrodepenetrates in the workpiece without translation.

During machining, the piston 1 is controlled to move the electrode andmake it penetrate into the workpiece. This movement is purely axial aslong as the piece 89 has not come to abut against the plate 93. However,from this moment, the rod 59 can no longer advance at the same time asthe plate 2 so that a relative axial displacement is produced betweenthe rod 59 and casing 55. The device of FIG. 5 then controls aneccentricity which increases linearly with the axial advance produced bypiston 1. To obtain a translational movement with a substantiallycircular trajectory, it is of course necessary to rotatably drive thepiece 64 by means of electric motor 42.

It is clear that in certain cases, a circular translational movement ofthe electrode may be undesirable. For example, when the cavity to bemachined must have in its lateral wall an angle or a dihedron having asharp angle in cross-section, a circular translation must not be used;to the contrary, it is advantageous to machine with a radial translationalong a given direction, preferably in a plane that bisects the sharpangle. This is obtained by stopping the motor 42 and angularly settingthe guide devices 68 parallel to said plane.

It should be remarked that although the described guide devices 68 areinclined by 45°, good results may be obtained with other angles ofinclination.

When translation is provided in a radial plane, i.e. when the motor 42does not drive piece 64, orientation of the radial plane is facilitatedby an arrangement, shown in FIG. 3, consisting of a stop rod 102slidably mounted in the cover 74 and having an end that can be placed inany one of a series of notches in the upper face of piece 64. Hence, thepiece 64 can be locked in a well determined angular position during themachining operation in question.

It is also clear that when carrying out the method it is not essentialto provide a purely axial advance of the electrode during the initialphase of machining. To the contrary, it is possible when carrying outthe preliminary adjustments to leave a certain eccentricity of thedevice, namely by the adjustment provided by means of nut 23 in the caseof FIG. 1, or that provided by the nut 71 in the case of FIG. 5. Hence,at the beginning of the finishing phase of machining, translationalmovements will be provided about the generatrices of a cone startingfrom points eccentric in relation to the axis of the cone.

I claim:
 1. In a method of machining by electrical discharges thesurfaces of a recess in a workpiece electrode to substantially the sameshape as a tool electrode, said method comprising controlling therelative displacements of the electrodes which are translationallymovable relative to one another both in the direction of an axis ofpenetration of one electrode into the other and in a plane perpendicularto said axis and displacing one of the electrodes along said axis ofpenetration to produce sparking in a machining zone comprised betweencorresponding surfaces of said electrodes, said machining zone extendingon frontal and lateral portions of said surfaces, the improvementcomprising simultaneously relatively moving the electrodes intranslation in said perpendicular plane with a predetermined amplitude,and progressively increasing said amplitude as a function of thepenetration along said axis to provide a combined oblique relativetranslational movement along at least one generatrix of a surface ofrevolution of increasing section in said direction of penetration, whilecontrolling said oblique relative translational movement to maintaingiven sparking conditions in the machining zone comprised between saidelectrodes at said frontal and lateral portions of said surfaces,wherein the amplitude of said relative translational movement in saidplane increases linearly with the penetration along said axis wherebysaid oblique relative translational movement is along a generatrix of acone coaxial to said axis of penetration and said generatrix issubstantially at 45° to said axis of penetration.
 2. The improvement ofclaim 1 in which said translational movement is started from the apex ofsaid cone at the beginning of a finish machining pass.
 3. Theimprovement of claim 2 in which prior to said finish machining pass saiddisplacement along said axis of penetration is combined with a relativetranslational movement of constant amplitude in said perpendicularplane.
 4. The improvement of claim 2 further comprising withdrawing saidone electrode along said axis of penetration with a simultaneousrelative translational movement in said perpendicular plane and ofprogressively decreasing amplitude, followed by a withdrawal of said oneelectrode along said axis when reaching the apex of said cone.
 5. Theimprovement of claim 1 further comprising providing a cyclic relativetranslational movement of the electrodes in said plane whereby saidrelative translational movement involves sweeping of said generatrixabout said cone surface.